Serum soluble IL-6 receptor levels during the mobilization of stem cells to peripheral blood.

Serum soluble interleukin-6 receptors (sIL-6R) have been demonstrated to play an important role in hematopoiesis. We report here that serum sIL-6R levels reflect proliferative kinetics of the progenitors after stimulation by chemotherapy plus granulocyte colony- stimulating factor. Serum sIL-6R were serially evaluated in 26 courses of peripheral blood (P13) stem cell collections in 16 patients using enzyme-linked immunosorbent assay. Expressions of IL-6R and CD34 on PB mononuclear cells were examined by flow cytometric analysis and expressions of IL-6R mRNA were examined by reverse transcriptase polymerase chain reaction. There were no significant differences between the serum sIL-6R levels on day 0 in patients (27.8 +/- 2.1 ng/ml, mean +/- SEM) and those in controls (27.5 +/- 1.5 ng/ml). Following chemotherapy the serum sIL-6R levels were significantly decreased, reaching a minimal level on day 14 (22.3 +/- 1.2 ng/ml, p < 0.01) and then significantly increased to above the baseline levels on day 21 (32.0 +/- 2.1 ng/ml, p < 0.01). Similar oscillations in the number of white blood cells, IL6R(+) cells, CD34(+) cells and colony-forming unit-granulocyte/macrophage (CFU-GM) in P13 could be observed and the peak expression of mRNA was compatible with the expression of antigen. Serum sIL-6R levels on day 17 and 19 were positively correlated with the number of CD34(+) cells, IL-6R(+) cells, CFU-GM in PB and the number of collected CD34(+) cells in leukapheresis products. In addition, when comparing the 2 groups divided by the number of prior chemotherapies, the status of disease or dose of the mobilizing regimen, the serum sIL-6R levels were significantly increased after day 17 in the group that received fewer courses of prior chemotherapy, the group in complete remission and the group of high-dose chemotherapy. These findings indicated that sIL-6R levels do not reflect the hematopoietic ability in the steady state, or the capability of the hematopoiesis after stimulation. Thus, sIL-6R levels may be a marker for the timing of PBSC collection or the prediction of the number of collected CD34(+) cells.